Field of the Invention
The present invention relates to spectroscopy, and more particularly, to an apparatus for spectroscopy of scattering light.
Description of the Prior Art
When an object to be examined is illuminated with light incident thereon, decomposition of scattering light from the object into its spectrum enables an investigator to obtain information about the thermodynamic and statistical properties of the object. When incident light is scattered by the object, Rayleigh scattering light, having the same frequency as that of the incident light, and Brillouin scattering light, having a shifted frequency from that of the incident light, can be observed. In order to investigate several properties of the object, it is necessary to observe the intensity of the spectrum, the shift of frequency, and the shape of the spectrum, which are obtained by decomposing the scattering light into its spectrum.
As is illustrated in FIG. 1, a reference numeral 1 is the Rayleigh scattering light which center frequency is the same as that of the incident light .nu..sub.o, and reference numeral 2 to 5 are the Brillouin scattering light which center frequencies shift from that of the incident light. As for spectroscopes, a Fabry-Perot interferometer is a spectroscope which directly decomposes light into its spectrum, and light beating spectroscopy electrically treats the light by making use of the beat of the light. Methods for treating signals of light beating spectroscopy include an analogue wave analysis and a digital photon-correlation technique. The Fabry-Perot interferometer has sufficient ability to measure the amount of the frequency shift of Brillouin scattering light, but does not have enough resolution to investigate the shape of each spectral line. Analogue wave analysis for treating signals of light beating spectroscopy may use, for example, a spectrum analyzer when the intensity of scattering light is great enough. However, since the scattering light is generally very weak, the signal which from a photo-multiplier is changed into an electric signal which is a pulse corresponding to photons so that photon-correlation spectroscopy is a suitable method in such cases. Direct data obtained by this photon-correlation spectroscopy method are intensity correlation functions of the scattered light, and when a plurality of spectra exist at the same time, those data are superpositions of the intensity correlation functions of each spectrum, so that it is difficult to separate and correspond it to each spectrum. Accordingly, the Fabry-Perot interferometer was mainly used to measure the amount of the shift of Brillouin scattering light, and the photon-correlation technique was used only to investigate the shape of Rayleigh scattering light when it had sufficient intensity.